Implement query result caching

[quinnjr]

Description

Add optional query result caching to improve performance for frequently accessed data.

Tasks

• Design cache interface trait
• Implement in-memory cache (LRU)
• Add Redis cache support
• Create cache middleware
• Add cache invalidation strategies
• Support TTL and manual invalidation
• Add cache statistics to metrics

Example Usage

let client = PraxClient::new(config)
    .with_cache(RedisCache::new(redis_url))
    .cache_ttl(Duration::from_secs(300));

// Cached query
let users = client.user()
    .find_many()
    .cache("active_users", Duration::from_secs(60))
    .exec().await?;

// Invalidate
client.cache().invalidate("active_users").await?;

Acceptance Criteria

• In-memory cache works
• Redis cache works
• Cache invalidation on writes
• Configurable TTL